Utilização de indicadores biogeoquímicos na reconstituição das mudanças paleoambientais do holoceno em área de transição de floresta/cerrado/campo (Humaitá - AM)

Abstract

The utilization of biogeochemical indicators on paleoenvironmental reconstruction at the Humaitá region (AM) helps estimate the frequency and magnitude of paleofires. Analysis of biogenic material flow, isotopes and radiocarbon allowed us to reconstruct (partially) the paleoclimatic conditions and the environmental scenario of the Humaitá region (AM) during the Holocene, the last years. The temporal interpretation regarding the changes biogenic material due to possible different climates indicated five climatic phases such as: 1- Dry phase with high fire frequency (4800 to 4600 years B.P.); 2- Wet phase (4600 to 4250 years B.P.); 3- Wet phase with high erosive activity and transition to dry phase (4250 to 3500 years B.P.), 4- Dry phase with low fire frequency (3500 to 400 years B.P.), and 5- Wet phase with high fire frequency (last 400 years B.P.). On the sediment core HUM 97/5 (Humaitá - AM), characterized by a presence of Rain Forest, Savanna and Wetland, we found high incidence of fires 300 years B.P.; with 78.700 particles flow cm-2 year1; and a second phase marked by low charcoal concentrations: 700 particles cm-2 year-1 in 1000 B.P.; 1.300 particles cm-2 year-1 in 1400 B.P.; 2.300 particles cm-2 year1 in 3850 years B.P. Higher values were detected on the top and the basal section of the core analyzed. The high levels of TOC showed values equivalent to the microcharcoal inputs and also the development of a biomass during the wet phase. However, the 13C values obtained on the sedimentary profiles analyzed (HUM 97/4 & HUM 97/5), indicated vegetation changes during the studied period, suggesting that the forest-savanna limit and the wetland might have suffered alterations during the Mid-Holocene to the Late-Holocene transition. In this case, the paleoclimatic scenario of the last 4800 years shows a paleovegetation ecotone predominantly herbaceous and arboreal, intermingled with Forest fragments due to edaphic factors, plus probable climate changes and resulting higher precipitation levels close to the recent climate mosaic